A human knee includes a lateral meniscus and a medial meniscus positioned between the joint surfaces of the femur and tibia. Each meniscus is a crescent or C-shaped piece of cartilage having a generally wedge-like or triangular cross-section that tapers inwards. In other words, the outer peripheral portion of each meniscus is relatively thick while the inner peripheral portion is relatively thin. Such an arrangement provides a “cup” for the curved bottom surface of the femur, which would otherwise move on the flat upper surface of the tibia with little joint stability. The menisci also play an important role in shock absorption and lubrication.
The outer peripheral portion of each meniscus is attached to the tibia by short coronary ligaments while the inner peripheral portion is not. Blood flows only to the outer peripheral portion from small arteries around the knee joint. As a result, the outer peripheral portion is sometimes referred to as a “red zone” and the remainder of the meniscus is referred to as a “white zone.” Because a meniscus is largely avascular (i.e., has a large white zone), it can be difficult for the body to heal tears not confined to the outer one-third of the meniscus.
The menisci may be torn when the knee experiences sudden and severe loads or as a result of degeneration. When a tear is located in the red zone, a surgeon may be able to repair it by suturing the tissue that forms the meniscus. More often, however, the torn portion requires removal because the tear is not limited to such an area. Repairing a small tear in the white zone of the meniscus is generally not effective because of the tissue's inability to heal even if sewn together. Typically, the portion of the meniscus with the tear is cut using small instruments and removed from the knee. The remainder of the meniscus is then balanced and contoured to provide a gradually tapered transition into the area of the resection.
Although some attempts have been made to replace the removed portion of a meniscus with an implant, there remains significant room for improvement. Many of the implants themselves have limited effectiveness, and the surgical procedures associated with them are often time-consuming, challenging, and not particularly suited for arthroscopic techniques.